The present invention relates generally to process monitoring in screw driven extruders, and more particularly to an ultrasonic probe and method for noninvasively monitoring materials processing in screw driven extruders.
Screw driven extruders are widely used in the manufacture of engineered plastics, polymer composites, and numerous components and products made therefrom. In screw driven extruders, the material being extruded is forced by the screw threads of the turning screw through the barrel of the extruder. The force of the screw heats the material. Thus, the material within the barrel may be in one of several different states, such as a solid state, a partially molten state, or a totally molten state, at different locations along the barrel of the extruder. Both process yield and quality may be improved by monitoring the state of the material being blended and processed between the extruder barrel and screw and using information generated thereby to control various process parameters, such as, for example, the rotational speed of the screw. However, the temperature, pressure and viscous shear required to drive the materials processing can severely limit the life of an invasive probe that can detect the state of the material.
Accordingly, there is a particular need for a probe and method of non-invasively monitoring the state of the material being extruded at selected locations along the barrel of a screw driven extruder.
In accordance with the present invention, there is provided a noninvasive probe for monitoring materials processing in a screw driven extruder. The probe comprises at least one ultrasonic transducer. The transducer is operable to transmit an ultrasonic signal on a signal path intersecting material between an inner sidewall of the barrel of the extruder and an outer surface of the screw within the barrel. Information about the state of the material intersected by the ultrasonic signal is ascertainable from an elapsed time between reception of a first reflection of the ultrasonic signal, resulting when the ultrasonic signal exits the inner sidewall of the barrel, and reception of a second reflection of the ultrasonic signal, resulting when the ultrasonic signal reaches the outer surface of the screw or partially solid material. For example, an elapsed time of a first duration A may indicate that the signal is intersecting completely molten material between the barrel and the screw. An elapsed time of a second duration B less than A may indicate that the signal is intersecting partially molten and partially solid material between the barrel and the screw. An elapsed time of a third duration C less than B may indicate that the signal is intersecting a thin layer of molten material in the gap between the barrel and the land of the screw.
In one embodiment of the present invention, a noninvasive probe for monitoring materials processing in a screw driven extruder comprises first, second and third ultrasonic transducers. The first, second and third transducers are operable to transmit first, second and third ultrasonic signals, respectively, on signal paths intersecting an inner sidewall of the barrel of the extruder and material between the inner sidewall of the barrel and an outer surface of the screw within the barrel. Information about the state of the material intersected by each ultrasonic signal is ascertainable from an elapsed time between reception of a first reflection of each ultrasonic signal and reception of a second reflection of each ultrasonic signal. The first reflections result when the ultrasonic signals exit the inner sidewall of the barrel and the second reflections result when the ultrasonic signals reach the outer surface of the screw or partially solid material.
In accordance with the present invention, there is provided a method of noninvasively monitoring material being processed in a screw driven extruder. The method comprises a first step of positioning at least one ultrasonic transducer for transmitting an ultrasonic signal on a signal path intersecting material between an inner sidewall of the barrel of the extruder and an outer surface of the screw within the barrel. An ultrasonic signal is then transmitted from the transducer. At least first and second reflections of the ultrasonic signal are then received. The first reflection results when the ultrasonic signal exits the inner sidewall of the barrel and the second reflection results when the ultrasonic signal reaches the outer surface of the screw or partially solid material. The state of the material intersected by the signal is then determined from the elapsed time between reception of the first and the second reflections.